Above threshold ionization in tightly focused,strongly relativistic laser fields

The dynamics of electrons ionized from high charge states by lasers with intensity >10(20) W/cm(2) have been studied. At these intensities vxB forces drive the electrons subsequent to ionization in a trajectory nearly parallel to the laser propagation direction. This gives rise to large energy gains as the electron rides in phase with the laser field over a long distance. Monte Carlo simulations illustrate that, unlike in case of ionization in sub- and near-relativistic intensity fields (<10(19) W/cm(2)), the electron dynamics in the ultrarelativistic case are strongly influenced by the longitudinal electric fields found near the focus of a tightly focused laser.     

Studies of plastic-ablator compressibility for direct-drive inertial confinement fusion on OMEGA

The compression of planar plastic targets was studied with x-ray radiography in the range of laser intensities of I approximately 0.5 to 1.5x10(15) W/cm2 using square (low-compression) and shaped (high-compression) pulses. Two-dimensional simulations with the radiative hydrocode DRACO show good agreement with measurements at laser intensities up to I approximately 10(15) W/cm2. These results provide the first experimental evidence for low-entropy, adiabatic compression of plastic shells in the laser intensity regime relevant to direct-drive inertial confinement fusion. A density reduction near the end of the drive at a high intensity of I approximately 1.5x10(15) W/cm2 has been correlated with the hard x-ray signal caused by hot electrons from two-plasmon-decay instability.     

Absorption of short laser pulses on solid targets in the ultrarelativistic regime

We report the first direct measurements of total absorption of short laser pulses on solid targets in the ultrarelativistic regime. The data show an enhanced absorption at intensities above 10(20) W/cm(2), reaching 60% for near-normal incidence and 80%-90% for 45 degrees incidence. Two-dimensional particle-in-cell simulations demonstrate that such high absorption is consistent with both interaction with preplasma and hole boring by the intense laser pulse. A large redshift in the second harmonic indicates a surface recession velocity of 0.035c.     

Photonuclear physics when a multiterawatt laser pulse interacts with solid targets

When a laser pulse of intensity 10(19) W cm(-2) interacts with solid targets, electrons of energies of some tens of MeV are produced. In a tantalum target, the electrons generate an intense highly directional gamma-ray beam that can be used to carry out photonuclear reactions. The isotopes 11C, 38K, (62,64)Cu, 63Zn, 106Ag, 140Pr, and 180Ta have been produced by (gamma,n) reactions using the VULCAN laser beam. In addition, laser-induced nuclear fission in 238U has been demonstrated, a process which was theoretically predicted at such laser intensities more than ten years ago. The ratio of the 11C and the 62Cu beta(+) activities yields shot-by-shot temperatures of the suprathermal electrons at laser intensities of approximately 10(19) W cm(-2).     

Probing nonequilibrium electron distributions in gold by use of second-harmonic generation

Second-harmonic radiation is generated at a gold surface by use of a laser pulse that is varied in duration from 14 to 29 fs and in intensity from 10(9) to 10(11)W/cm(2) . At laser intensities below 10(10)W/cm(2) , the second-harmonic signal has the expected quadratic dependence on pump-laser intensity; however, at higher intensities, the dependence is supraquadratic. This difference arises because the leading edge of the laser pulse interacts significantly with the gold electrons to create a nonequilibrium, photoexcited distribution. The second-harmonic generation process occurs before electron-electron or electron-phonon collisions can equilibrate the distribution and therefore serves as a probe of the nonequilibrium distribution.     

Laser-driven generation of fast particles

The great progress in high-peak-power laser technology has resulted recently in the production of ps and subps laser pulses of PW powers and relativistic intensities (up to 10²¹ W/cm²) and has laid the basis for the construction of multi-PW lasers generating ultrarelativistic laser intensities (above 10²³ W/cm²). The laser pulses of such extreme parameters make it possible to produce highly collimated beams of electrons or ions of MeV to GeV energies, of short time durations (down to subps) and of enormous currents and current densities, unattainable with conventional accelerators. Such particle beams have a potential to be applied in numerous fields of scientific research as well as in medicine and technology development. This paper is focused on laser-driven generation of fast ion beams and reviews recent progress in this field. The basic concepts and achievements in the generation of intense beams of protons, light ions, and multiply charged heavy ions are presented. Prospects for applications of laser-driven ion beams are briefly discussed.     

Role of hot-electron preheating in the compression of direct-drive imploding targets with cryogenic D2 ablators

The compression of direct-drive, spherical implosions is studied using cryogenic D2 targets on the 60-beam, 351-nm OMEGA laser with intensities ranging from approximately 3x10(14) to approximately 1x10(15) W/cm2. The hard-x-ray signal from hot electrons generated by laser-plasma instabilities increases with laser intensity, while the areal density decreases. Mitigating hot-electron production, by reducing the laser intensity to approximately 3x10(14) W/cm2, results in areal density of the order of approximately 140 mg/cm2, in good agreement with 1D simulations. These results will be considered in future direct-drive-ignition designs.     

Validation of thermal-transport modeling with direct-drive, planar-foil acceleration experiments on OMEGA

We present for the first time the experimental validation of the nonlocal thermal-transport model for a National Ignition Facility relevant laser intensity of approximately 10(15) W/cm(2) on OMEGA. The measured thin target trajectories are in good agreement with predictions based on the nonlocal model over the full range of laser intensities from 2 x 10(14) to 10(15) W/cm(2}) The standard local thermal-transport model with a constant flux limiter of 0.06 disagrees with experimental measurements at a high intensity of approximately 10(15) W/cm(2) but agrees at lower intensities. These results show the significance of nonlocal effects for direct-drive ignition designs.     

Photonuclear fission from high energy electrons from ultraintense laser-solid interactions

A new regime of laser-matter interactions in which the quiver motion of plasma electrons is fully relativistic, with energies extending well above the threshold for nuclear processes, is studied using a petawatt laser system. In solid target experiments with focused intensities exceeding 10(20) W/cm(2), high energy electron generation, hard bremsstrahlung, and nuclear phenomena have been observed. We report here a quantitative comparison of the high energy electrons and the bremsstrahlung spectrum, as measured by photonuclear reaction yields, including the photoinduced fission of 238U.     

Scaling hot-electron generation to high-power, kilojoule-class laser-solid interactions

Thin-foil targets were irradiated with high-power (1 ≤ P(L) ≤ 210 TW), 10-ps pulses focused to intensities of I>10(18) W/cm(2) and studied with K-photon spectroscopy. Comparing the energy emitted in K photons to target-heating calculations shows a laser-energy-coupling efficiency to hot electrons of η(L-e) = 20 ± 10%. Time-resolved x-ray emission measurements suggest that laser energy is coupled to hot electrons over the entire duration of the incident laser drive. Comparison of the K-photon emission data to previous data at similar laser intensities shows that η(L-e) is independent of laser-pulse duration from 1 ≤ τ(p) ≤ 10 ps.     

Radiation-pressure acceleration of ion beams from nanofoil targets: the leaky light-sail regime

A new ion radiation-pressure acceleration regime, the "leaky light sail," is proposed which uses sub-skin-depth nanometer foils irradiated by circularly polarized laser pulses. In the regime, the foil is partially transparent, continuously leaking electrons out along with the transmitted laser field. This feature can be exploited by a multispecies nanofoil configuration to stabilize the acceleration of the light ion component, supplementing the latter with an excess of electrons leaked from those associated with the heavy ions to avoid Coulomb explosion. It is shown by 2D particle-in-cell simulations that a monoenergetic proton beam with energy 18 MeV is produced by circularly polarized lasers at intensities of just 101? W/cm2. 100 MeV proton beams are obtained by increasing the intensities to 2 × 102? W/cm2.     

Laser-cluster interaction: x-ray production by short laser pulses

We investigate the heating of the quasifree electrons in large rare-gas clusters (N exceeding 10(5) atoms) by short laser pulses at moderate intensities (I approximately = 10(15) W cm(-2)). We identify elastic large-angle backscattering of electrons at ionic cores in the presence of a laser field as an efficient heating mechanism. Its efficiency as well as the effect of collective electron motion, electron-impact ionization, and cluster charging are studied employing a mean-field classical transport simulation. Results for the absolute x-ray yields are in surprisingly good quantitative agreement with recent experimental results.     

Time-resolved measurements of hot-electron equilibration dynamics in high-intensity laser interactions with thin-foil solid targets

Time-resolved K(α) spectroscopy has been used to infer the hot-electron equilibration dynamics in high-intensity laser interactions with picosecond pulses and thin-foil solid targets. The measured K(α)-emission pulse width increases from ~3 to 6 ps for laser intensities from ~10(18) to 10(19) W/cm(2). Collisional energy-transfer model calculations suggest that hot electrons with mean energies from ~0.8 to 2 MeV are contained inside the target. The inferred mean hot-electron energies are broadly consistent with ponderomotive scaling over the relevant intensity range.     

Saturation of the two-plasmon decay instability in long-scale-length plasmas relevant to direct-drive inertial confinement fusion

Measurements of the hot-electron generation by the two-plasmon-decay instability are made in plasmas relevant to direct-drive inertial confinement fusion. Density-scale lengths of 400 μm at n(cr)/4 in planar CH targets allowed the two-plasmon-decay instability to be driven to saturation for vacuum intensities above ~3.5×10(14) W cm(-2). In the saturated regime, ~1% of the laser energy is converted to hot electrons. The hot-electron temperature is measured to increase rapidly from 25 to 90 keV as the laser beam intensity is increased from 2 to 7×10(14) W cm(-2). This increase in the hot-electron temperature is compared with predictions from nonlinear Zakharov models.     

Measurements of energetic proton transport through magnetized plasma from intense laser interactions with solids

Protons with energies up to 18 MeV have been measured from high density laser-plasma interactions at incident laser intensities of 5x10(19) W/cm(2). Up to 10(12) protons with energies greater than 2 MeV were observed to propagate through a 125 &mgr;m thick aluminum target and measurements of their angular deflection were made. It is likely that the protons originate from the front surface of the target and are bent by large magnetic fields which exist in the target interior. To agree with our measurements these fields would be in excess of 30 MG and would be generated by the beam of fast electrons which is also observed.     

Intensity limits for propagation of 0.527 microm laser beams through large-scale-length plasmas for inertial confinement fusion

We have established the intensity limits for propagation of a frequency-doubled (2omega, 527 nm) high intensity interaction beam through an underdense large-scale-length plasma. We observe good beam transmission at laser intensities at or below 2x10(14) W/cm(2) and a strong reduction at intensities up to 10(15) W/cm(2) due to the onset of parametric scattering instabilities. We show that temporal beam smoothing by spectral dispersion allows a factor of 2 higher intensities while keeping the beam spray constant, which establishes frequency-doubled light as an option for ignition and burn in inertial confinement fusion experiments.     

Flux-limited nonequilibrium electron energy transport in warm dense gold

An abrupt change in energy transport has been observed in femtosecond laser heated gold when the absorbed laser flux exceeds ~7×10(12) W/cm(2). Below this value, the absorbed flux is carried by ballistic motion of nonthermal electrons produced in interband excitation. Above this value energy transport appears to include ballistic transport by nonthermal electrons and heat diffusion by thermalized hot electrons. The ballistic component is limited to a flux of ~7×10(12) W/cm(2). This offers a unique benchmark for comparison with theory on nonequilibrium electron transport.     

Few-photon multiple ionization of ne and ar by strong free-electron-laser pulses

Few-photon multiple ionization of Ne and Ar atoms by strong vacuum ultraviolet laser pulses from the free-electron laser at Hamburg was investigated differentially with the Heidelberg reaction microscope. The light-intensity dependence of Ne2+ production reveals the dominance of nonsequential two-photon double ionization at intensities of I<6x10(12) W/cm2 and significant contributions of three-photon ionization as I increases. Ne2+ recoil-ion-momentum distributions suggest that two electrons absorbing "instantaneously" two photons are ejected most likely into opposite hemispheres with similar energies.     

Decay of cystalline order and equilibration during the solid-to-plasma transition induced by 20-fs microfocused 92-eV free-electron-laser pulses

We have studied a solid-to-plasma transition by irradiating Al foils with the FLASH free electron laser at intensities up to 10(16) W/cm(2). Intense XUV self-emission shows spectral features that are consistent with emission from regions of high density, which go beyond single inner-shell photoionization of solids. Characteristic features of intrashell transitions allowed us to identify Auger heating of the electrons in the conduction band occurring immediately after the absorption of the XUV laser energy as the dominant mechanism. A simple model of a multicharge state inverse Auger effect is proposed to explain the target emission when the conduction band at solid density becomes more atomiclike as energy is transferred from the electrons to the ions. This allows one to determine, independent of plasma simulations, the electron temperature and density just after the decay of crystalline order and to characterize the early time evolution.     

Photofragmentation of Na2+ in intense femtosecond laser fields: from photodissociation on light-induced potentials to field ionization.

Photofragmentation of Na2 + molecules in well prepared vibrational levels has been studied employing intense ( 10(11)-10(14) W/cm2) and ultrashort (80 fs) 790 nm laser fields. Four fragmentation channels with different released kinetic energies are observed. Depending on the applied laser intensity, the fragmentation of Na2 + is governed by photodissociation on light-induced potentials and field ionization followed by Coulomb explosion. Below 1x10(12) W/cm2, only photodissociation on light-induced potentials is seen. For intermediate laser intensities, field ionization at large internuclear distances competes with photodissociation, thus preventing the observation of above threshold dissociation. Field ionization at small internuclear distances dominates for the highest laser intensities used.